Method and apparatus for testing powdered compacts

ABSTRACT

A METHOD AND APPARATUS FOR TESTING UNSINTERED COMPACTED POWDER ARTICLES FOR STRUCTURAL FLAWS BY PASSING A FLUID UNDER PRESSURE INTO REGIONS OF THE ARTICLE WHERE FLAWS ARE MOST LIKELY TO OCCUR. THE FLUID CAN BE APPLIED UNDER A CONSTANT STANDARD PRESSURE OR A VARIABLE PRESSURE WHICH IS INCREASED UP TO THE STANDARD PRESSURE SUCH THAT FLAWED ARTICLES FAIL AND UNFLAWED ARTICLES REMAIN INTACT. THE STANDARD PRESSURE IS ESTABLISHED EMPIRICALLY FOR A GIVEN COMPACT IN RELATIONSHIP TO THE MAXIMUM PRESSURE THAT CAN BE APPLIED WITHOUT PRODUCING FAILURE TO UNFLAWED   COMPACTS MADE FROM THE SAME COMPOSITION AND BY THE SAME METHOD. THE COMPACT DURING TEST IS CLAMPED IN A JIG ADAPTED SO THAT FLUID PRESSURE CAN BE APPLIED TO PREDETERMINED REGIONS WHERE FLAWS ARE SUSPECTED AND SO DESIGNED THE COMPRESSIVE CLAMPING FORCE IS NOT APPLIED IN OPPOSITION TO THE TENSILE FORCE ON THE FLAW REGION GENERATED BY FLUID.

Nov. 9, 1971 HAWKES 3,618,364

METHOD AND APPARATUS FOR TESTING POWDERED COMPACTS Filed Sept. 2, 1969 3 Sheets-Sheet 1 INVENTOR IVOR HAWKES i-MJ/j/ (ATTORNEYS Nov. 9, 1971 1. HAWKES 3,618,364

METHOD AND APPARATUS FOR TESTING POWDERED COMPACTS Filed Sept. 2, 1969 3 Sheets-Sheet 3 INVENTOR IVOR HAWKES gar/ E J ATTOR NEYS Nov. 9, 1971 HAWKES 3,618,364

METHOD AND APPARATUS FOR TESTING POWDERED COMIACTS Filed Sept. 23, 1969 5 Shoots-Shoot 5 INVENTOR IVOR HAWKES ATTORNEYS United States Patent Office 3,618,364 Patented Nov. 9, 1971 3,618,364 METHOD AND APPARATUS FOR TESTING POWDERED COMPACTS Ivor Hawkes, Lyme, N.H., assignor to Creare Inc., Hanover, N.H. Filed Sept. 2, 1969, Ser. No. 854,589

Int. Cl. G01m 3/26 US. CI. 73-37 9 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for testing unsintered compacted powder articles for structural flaws by passing a fluid under pressure into regions of the article where flaws are most likely to occur. The fluid can be applied under a constant standard pressure or a variable pressure which is increased up to the standard pressure such that flawed articles fail and unflawed articles remain intact. The standard pressure is established empirically for a given compact in relationship to the maximum pressure that can be applied without producing failure to unflawed compacts made from the same composition and by the same method. The compact during test is clamped in a jig adapted so that fluid pressure can be applied to predetermined regions where flaws are suspected and so designed that the compressive clamping force is not applied in opposition to the tensile force on the flaw region generated by the fluid.

This invention relates to a method and apparatus for testing stable articles formed from compacted unsintered powders, more particularly articles formed from metallic powders for structural flaws.

The standards for determining whether an unsintered compact or sintered article produced therefrom are acceptable primarily depend upon the end use of the sintered article and the standards of appearance and structural strength of the sintered article in that end use. Accordingly, the terms flaw, flawed and unflawed as used herein are relative and not absolute and depend upon the acceptability of article in its end use.

To form stable sintered articles from metallic powder, the powder is first compacted into the shape of the finally desired product and after compacting is sintered by heating. To prevent flaws such as cracks or high surface porosity in sintered parts, the unsintered compact must be free of flaws. Flaws in the compact usually take the form of either local regions of high porosity or slip planes and these flaws are generally retained even after the compact is sintered. When the flaw exists as a region of local high porosity, the number and area of grain to grain contacts is less per unit area than in the remainder of the compact. For a given load, the stress at these grain to grain contacts in the high porosity region will therefore be higher than in the remainder of the compact and failure will occur in these regions at lower loads. When the flaws exist as slip planes, the tensile strength of the compact across the slip plane will be virtually zero due to the physical separation which occurs between the original grain to grain contacts during the generation of the slip planes. Under such circumstances, relatively low tensile loads on the sintered article will cause failure.

The compact flaws can arise for several reasons, but mainly because of the inability of the powder to flow freely during the pressing or compacting operation. The mechanics of punch and die movements and powder compacting are fairly well understood and it can be shown that for example in compacts having a stepped shape, the regions of high porosity or slip plane flaws occur mainly in the vicinity of the inside corners where abrupt changes of cross-sectional area occur. In practice, these flaws often occur as a result of changes in the optimum punch and die movement and as a result of minute changes in the homogeniety of the powdered material. At the present time, there are no available practical methods for testing the tensile strength of unsintered compacts made from powdered materials. The usual mechanical strength tests described in MPIF Standard 35-65 are designed for evaluating powder mixes and compacting pressure and cannot be used for testing compacts for strength at specific points in the compact. It would be highly desirable to provide a reliable means for testing unsintered powdered compacts for strength in order to avoid flaws in the sintered articles prepared therefrom.

In accordance with the present invention there is pro vided an apparatus and method for testing unsintered powder compacts for structural flaws. A fluid, gas or liquid, under pressure is directed into a compact at predetermined regions where theoretically and practically flaws develop when the compact is sintered. These regions can be easily identified on the basis of knowing how the compacts are made as for example, by the procedure discussed by H. G. Taylor in the article, The Influence of Tooling Methods on the Density Distribution in Complex Metal Powder Parts, Powder Metallurgy 1960, No. 6, p. 87-124.

Alternatively, these regions can be determined by examining the sintered compacts for appearance or by sectioning the sintered compacts to determine the nature and position of the flaws, when they exist. When the flaw region or regions have been determined for a given compact and pressing technique, the fluid is introduced at a constant standard pressure or a pressure which varies up to the standard pressure to generate a tensile force across the suspected flaw region by the internal pore pressure. Under the applied pressure, flawed compacts fail but unflawed compacts are unaffected.

The fluid is introduced into the compact in a direction to establish a pressure differential across the most likely flaw region and thereby providing a tensile force at the flaw in a direction substantially normal to the flaw. The compact is held in a jig constructed to enable the fluid pressure to be applied only over the limited area comprising the predetermined flaw regions and to permit fractrue of the flawed compacts across the flaw region under the force caused by the introduced fluid. That is to say, the jig does not establish a compressive force in opposition to the tensile force caused by the introduced fluid and therefore does not prevent failure of a flawed compact. Under certain circumstances, it may be necessary to clamp a region of the compact which is unflawed but is fragile or has a relatively small cross-sectional area to prevent failure in the unflawed region while testing for flaws in other regions of the compact. As the fluid flows through the compact, pressure and force gradients will be established and their magnitude will depend on the resistance to fluid flow. When a fluid is directed into a flaw there is little pressure drop into the compact because the flaw structure does not restrict fluid flow. The greatest force gradient in the compact will therefore occur normal to the plane of the flaw structure having the least grain to grain contacts and failure will occur here at a substantially lower fluid pressure than in a similar but unflawed compact. Tests have shown that the pressures required to cause failure of flawed compacts ranges from 10 p.s.i. to over 500 psi. depending upon the dimensions, and composition of the compact. Generally, the pressure needed to cause failure for a flawed compact will be less than about 50 percent of the pressure needed to cause failure of the unflawed compact The standard pressure for a given batch of compacts is the minimum fluid pressure employed to fracture compacts such that all the compacts remaining intact after applying the fluid pressure are unflawed after sintering. In practice, the standard pressure must be established for a given powder composition compacted by a particular compacting process since the value obtained is not applicable to other types of compacts. Furthermore, different standard pressures are established for different regions in the compact where flaws are suspected since the fluid pressure at which an unflawed region of the compact will fracture depends upon the size and shape of that region. In practice, the standard pressure for a particular test process can be established very close to the theoretical standard pressure by any one of a number of alternative methods.

Thus, the standard test pressure for a given type of compact is determined empirically in relationship to the maximum pressure needed to fracture an unflawed compact. The maximum fluid pressure can be determined from a batch of compacts taken from a production run where all the variables in the compacting process have been controlled as accurately as possible. One-half of the batch can be tested to determine the maximum fluid pressure that will cause failure across the suspected flaw region. The remaining compacts are sintered and thereafter thoroughly checked visually and by cutting them into small pieces to determine whether or not any are flawed. If the powder mix, differential press movements, compacting pressures and sintering cycle all operate as designed, then all the sintered parts should be satisfactory. If not, the necessary adjustments in the compacting process should be made and the whole operation repeated, in luding the pressure tests on the compacts. Thus, the present invention is primarily useful to test compacts formed in a compacting process which has deviated from the optimum. Once it has been ascertained that statistically all of the compacts in the batch are satisfactory, then the highest pressure required to fail a compact can be taken as the maximum pressure and the scatter in pressure measurements as the operating range. The relationship between the standard test pressure and the maximum pressure is determined statistically and by experience but will be governed to a great extent by the spread of the pressure measurements made on the test batch. For example, the standard pressure can be 2 or 3 percent lower than the lowest pressure measurement in the test batch when the pressure measurement spread is not more than about 5 percent. During subsequent production runs if numerous compacts fail at the chosen standard pressure, then the whole procedure can be repeated to determine if the pressure was initially set too high.

Alternatively, the standard pressure can be established empirically by subjecting compacts to varying fluid pressures and then sintering the compacts remaining intact after application of fluid pressure to determine which remain unflawed after sintering. Knowing the particular fluid pressure previously applied to the compacts and the proportion of compacts remaining unflawed during the subsequent sintering operation, a second set of compacts made from the same powder composition by the same process is subjected to either a higher or a lower fluid pressure than previously employed. A lower fluid pressure is employed when all of the compacts remain unflawed during sintering since there is a strong likelihood that the fluid pressure previously employed was excessive for purposes of testing and that there would be increased numbers of compacts which would have remained unflawed during the sintering step. On the other hand, when the proportion of flawed compacts in the sintering step is excessive, this indicates that the fluid pressure previously employed was excessively low for use as the standard pressure, In the latter instance, the fluid pressure employed during the second testing step is increased. The successive application of fluid pressure followed by sintering is repeated until a minimum fluid pressure is established whereby all of the compacts remaining intact during the testing step also remain unflawed after the sintering step.

Alternatively, the standard pressure for a particular compacting process and powder composition can be varied within relatively narrow limits. The initial standard pressure for the compacts can be established, as for example by the procedures described above, for compacts formed by a compacting process operating under the optimum design conditions. However, the standard pressure can change due to slight changes in the compacting process which are still within the design limits. Thus, by noting the trend, if any, in the pressure needed to fracture the compacts, the standard pressure can be revised up or down in accordance with the trend. This procedure takes advantage of the fact that a condition in a compacting process can vary but still be maintained within design limits.

The comparative method for testing compacts in accordance with the process of this invention rather than an absolute method for testing compacts is most desirable since the strength and porosity of compacted powder varies with the shape of the compact with different compositions and with the particular compacting method employed.

The apparatus provided by the present invention comprises a means for generating fluid pressure, usually air or another gas, a means for controlling and measuring the fluid pressure including the pressure at which failure occurs, if any, and a jig or fixture designed to hold the compact without the application of a compression force in opposition to the tensile force normal to the flaw caused by the fluid introduced therein.

The particular jig construction employed depends upon the direction in which the flaw extends but in any event does not exert a compressive clampive force in opposition to the tensile force caused by the fluid except where this would cause the compact to fail in an unflawed region. The jig comprises a solid structure having a hollow chamber of a cross-section adapted to fit the compact. The jig has at least one conduit extending therethrough to direct fluid through an orifice into the chamber at the point where the pressure is to be applied to the compact. For example, when testing a stepped compact where the suspected flaw region is in the vicinity of the internal corner, one orifice may be located at the junction of the larger and smaller cross-section of the chamber. Seals are provided around the orifice to prevent leakage between the surfaces of the compact and the internal walls of the jig. When testing radially symmetrical compacts, the seals form an annular chamber around the compact. The jig can be designed to enable a compact to be tested in several ways for flaws which penetrate into different sections of the compact. For example, a flaw structure in a radially symmetrical flanged or stepped compact could penetrate either into the main body of the compact (FIG. 1c) or into the flange from the internal corner region (FIG. 1b). To test for the flaw which penetrates the main body of the compact, the compact is clamped into the jig between the upper face of the jig and the flange so that the lower face of the main compact body is free as shown in FIG. 2. Fluid applied under pressure to the corner region, as shown in FIG. 2 will cause a pressure gradient to be set up between the internal corner region and the bore of the compact.

Alternatively, when it is desired to test the compact for flaws in the flange section only, the jig maybe constructed so that it contacts the whole outside surface of the compact with the exception of the bottom and side surfaces of the flange section. There being no compressive force on the flange in opposition to the fluid tensile force, a flawed flange will fail upon introduction of the fluid under pressure. This jig construction is preferred when testing for flaws in the flange since there are minimal frictional forces caused by contact between the jig and bottom flange surface. The flange need only be raised from the jig surface a few thousandths of an inch, location of the flange underface with respect to the jig surface being maintained by locating member 33 (FIG. 4), and sealing being attained by slight compression of an O ring or other sealing member.

FIG. 1 is an isometric view of a typical stepped powder compact;

FIG. 1a is a detailed sectional view of the compact of FIG. 1 showing a typical high porosity type flaw;

FIG. 1b is a detailed sectional view of the compact of FIG. 1 showing a slip plane flaw penetrating into the flange;

FIG. is a detailed view of the compact of FIG. 1 showing a slip plane flaw in the main body of the compact;

FIG. 2 is a schematic view of one embodiment of the apparatus for testing compacts for flaws in the main body of the compact;

FIG. 3 is a schematic view of the apparatus employed to test the flange section;

FIG. 4 is a schematic diagram of another embodiment of the apparatus of this invention; and

FIG. 5 is a cross sectional view of the jig used to test two areas of the compact simultaneously.

As shown in FIG. 1, a typical compact may be comprised of a flange section 2 and a body section 3 having a hollow channel 4 through the center thereof. Flaws are most likely to occur in area 5 at the intersection of the flange 2 and the main body 3. As shown in FIGS. 1a, 1b and 1c, the flaws can be any of three general types. As shown in FIG. 1a, the flaw 6 at the intersection of flange 2 and main 'body 3 is an area of high porosity. As shown in FIG. 1b, the flaw 7 at the intersection of the flange 2 and the main body 3 is a slip plane extending into the flange 2. As shown in FIG. 10, the flaw 8 at the intersection of flange 2 and main body 3 is a slip plane extending into the main body 3.

For testing for flaws extending into the main body (as in FIGS. 1a and 1c) as shown in FIG. 2, the compact 1 is placed into the recess 9 of the jig 10 in a manner so that the bottom surface 11 of the compact 1 and the sides I12 of the flange 2 are not in contact with the jig 10. Thus, the body 3 is not subjected to compressive forces opposed to any tensile forces to which the compact 1 may be subjected while the flange 2 is subjected to only a minimal compressive force caused by friction between the jig and the surface 15.

A fluid under pressure is directed through conduits 13 and 14 through openings 16 to region 5 of the compact 1. The gas introduced through conduit 13 is retained in space 17 which extends around the entire inside peri hery of the jig by sealing means 18 surrounding the orifice 16 to direct the gas through the compact 1 rather than between the compact and the jig. The compact 1 is clamped into the jig 10 by plate 19 to which is applied a compressive force to prevent its ejection under the applied gas pressure. Pressure gauge 21 measures the fluid pressure in conduit 13 applied to the compact 1 while valve or constriction 22 and regulator 23 control the fluid flow rate and pressure, member 22 also acting as a restric tion to gas flow so that if the compact fails, the pressure on the compact 1, as recorded by the gauge 21, will fall suddenly to about 10 percent of the gas pressure shown prior to failure.

The jig I10 of FIG. 2 is designed to test primarily for slip plane flaws penetrating into the body 3. The total gas force acting on any flaw in this region depends upon its orientation and the pressure drop between surface 30 and surface 31 through the compact '1. When a pressure differential across the compact 1 is elevated to the point where compact failure is attained, failure will occur in the body. The jig 10 of FIG. 2 also can be used to detect slip plane flaws penetrating from the corner region 5 into the flange 2. However, these latter slip plane flaws may best be detected by the embodiment of this invention described with reference to FIG. 3.

Referring to FIG. 3 a jig 32 is provided with a vertically adjustable bottom plate 33 which contacts the bottom surface 34 of compact 1. A compressive force on plate 19 clamps compact 1 into recess 35 of jig 32 and together with the compressive force of bottom plate 33 exerts a compressive force in opposition to any tensile force in the main body 1 caused by the fluid. This feature is necessary only if the rupture pore pressure of the main body 1 is lower than that of the flange 2 due to a flaw or to its inherent lower strength. The fluid is introduced through conduit 36 and orifice 37 into space 38. The fluid is retained under pressure in space 38 by sealing means 39 which extend above surfaces 40' and 41. The flange section 2 of compact :1 does not contact surface 41 of jig 32. Thus, there is minimal compressive force and hence resistive shearing force exerted on flange section 2 to counteract the fluid tensile force so that flawed flange sections will fail under the fluid tensile force.

Referring to FIG. 4 refinements to the gas flow circuit shown in FIG. 2 are shown for the purpose of speeding testing, giving a permanent record of the failure pressure and giving an electrical signal to indicate a compact of low strength. The pressure regulator 25 placed in parallel relationship with the flow control valve 22 is used to apply a standard pressure, established empirically as described above, through fluid conduit 26 to the compact 1. The pressure can be set so that only flawed compacts having a low tensile strength will fail. When the shutoff valve 27 in series with the by-pass regulator 25, is closed, the pressure on the compact side of the control valve 22 will fall or remain steady depending on the restriction of the valvs 25 and whether or not the compact has failed. A pressure switch 28 set to operate at the standard pressure and actuated when the valve 27 is closed is adapted to give an electrical indication of a failed or unfailed compact. By suitable adjustment of the feed regulator 23, the by-pass regulator 25, and the control valve 22, applied pressure to the compact can be varied up to any fixed limit and the pressure can be recorded by a pressure strip chart recorder 29 or other suitable recording means. With this arrangement, failure of the compact under the force of the fluid applied at the standar pressure can readily be determined by a sudden drop in pressure indicated by the pressure switch 28.

Any sealing means can be employed to retain the fluid under pressure in the compact region where flaws are most likely to occur. Thus, 0 rings, inflated seals and the like can be employed.

Referring to FIG. 5, the compact 50 is retained in jig 51 by means of pressure being exerted on plate 52. The jig 51 is shaped so that there is a space between its bottom surface 53 and the bottom surface 54 of recess 55. Seals 56 and 57 'direct gas entering through conduit 58 into area 59 of the compact 50. Seals 57 and 60 direct gas entering through conduit 61 into area 62 of the compact 50. Any means to prevent pressure buildup in the recess 55 can be employed, such as a port in the wall in communication with the outside atmosphere.

In testing compacts in accordance with this invention, the compacts can be sequentially tested for flaws in a plurality of sections such as the flange section and body section by sequentially subjecting them to the standard pressure in a plurality of jigs adapted to direct pressure in different sections such as described with reference to FIG. 2 and FIG. 3. Furthermore, this invention can be employed to test compacts having any shape including those having more than two diiferent cross-sections by providing a jig having a recess shaped to accomodate the compact. These jigs have a plurality of fluid supply conduits and seals, each adapted to deliver fluid under pressure to regions where flaws are suspected.

While this invention has been described specifically with reference to metallic powder compacts having a stepped cross-section it is to be understood that the invention is not limited thereto. Thus, this invention is applicable to all powder compacts regardless of powder composition and shape. Accordingly, the invention is applicable for example to creamic powders as Well as metallic powders and is applicable to symmetrical compacts including cylindrical compacts or irregularly shaped compacts including those having an irregular shape such as gears or perforations and may be hollow or solid. All that is required is that the jig have a hollow chamber adapted to receive the compact, be adapted to compress the regions of the compact not being tested to prevent failure in unflawed regions, be adapted not to exert a compressive force in opposition to the fluid tensile force in the test region and have one or a plurality of means for delivery and sealing fluid in those compact regions under test. To meet the requirements, the jig can be nonadjustable or adjustable, either vertically or horizontally to vary the shape and dimension of the hollow chamber. This will, of course, depend upon the shape of the compact being tested.

Furthermore, while the process of this invention has been described specifically with reference to the use of a constant standard pressure to test the compacts, it is to be understod that the invention includes the use of a variable pressure to test the compacts. Thus, under the forces generated by a constant standard pressure, the compact will either fracture or remain intact, and the acceptability of the compact can be easily determined. Alternatively, a variable pressure up to the standard pressure can be employed and the pressure at which failure, if any, occurs can be noted and compared to the standard pressure to determine compact acceptability. In another embodiment, the compact can be subjected to a variable pressure which may exceed the standard pressure to effect fracture of all compacts tested and compare the pressure at which fracture occurs with the standard pressure to determine compact acceptability. Of course, since the latter method is a totally destructive test, only a proportion of the compacts are tested which proportion is determined by well known statistical techniques.

I claim:

1. A jig for testing unsintered compacted powder articles for structural flaws comprising a body having a hollow chamber to accomodate an article, means for delivering fluid under presure to said chamber, sealing means to confine said fluid at a region of the article in said chamber without exerting a compresisve force in opposition to the fluid tensile force generated by the delivered fluid in the flaw region.

2. The jig of claim 1 wherein at least one surface of the hollow chamber is movable to permit changing the size or shape of the hollow chamber.

3. The jig of claim 1 having a plurality of means for simultaneously delivering fluids and sealing means to confine fluid at a plurality of suspected flaw regions of the article.

4. A process for testing compacted powdered articles produced from a common compacting'process for structural flaws which comprises establishing a standard pressure at which all flawed articles fracture and unflawed articles remain intact, applying a fluid at the established standard pressure to a region of the article where flaws are suspected, said fluid being confined on the surface of the article adjacent said region whereby a tensile force is established at a flaw suflicient to fracture flawed articles, said fluid being confined and applied, without a compression force being exerted in opposition to said tensile force in said region.

5. The process of claim 4 wherein the articles is sequentially tested at different suspected flaw regions.

6. The process of claim 4 wherein the article is simultaneously tested at different suspected flaw regions.

7. The process of claim 4 wherein the article tested has an unflawed region which will fail under the fluid tensile force generated at or less than the standard pressure wherein a compressive force is applied to the unflawed region to prevent failure therein.

8. The process of claim 4 wherein the fluid under pressure is applied to the article at a variable pressure, up to the standard pressure.

9. A process for testing permeable articles for strength which comprises applying a fluid under pressure to a surface to establish a pressure gradient and cause fluid flow within and through the article from the pressurized region to a second surface of the article being maintained at a lower pressure, increasing the fluid pressure until said gradient is sufficient to establish by internal pore pressure a tensile force sufiicient to fracture the article, and measuring the pressure at which fracture occurs.

References Cited UNITED STATES PATENTS 756,644 4/1904 Johnson 73-37QUX 2,222,079 11/1940 Larson 73-37 3,413,839 12/1968 Clark et al. 73-37 LOUIS R. PRINCE, Primary Examiner W. A. HENRY, Assistant Examiner U.S. Cl. X.R. 7338 

